Modélisation éléments finis et prédiction personnalisées de la progression de l’anévrisme aortique

La paroi aortique, comme d’autres systèmes biologiques, présente des adaptations visant à maintenir sa stabilité. Cette stabilité dépend de l’évaluation de l’état de l’aorte par la cellule. A terme, une activité perturbée de les cellules peut conduire à des inadaptations et à la progression de maladies. Ainsi, l’objectif de cette thèse est d’implémenter une approche mécanique des adaptations dans un solveur numérique et de l’appliquer à des aortes spécifiques aux patients. Tout d’abord, l’approche mécanique est incluse dans une méthode de coque axisymétrique bidimensionnelle. La croissance et le remodelage (G&R) du tissu sont déclenchés par l’élimination de la masse ou les changements de charge. Les changements de charge sont produits par le placement d’un stent dans l’artère et produisent un remodelage supplémentaire de celle-ci. Deuxièmement, le modèle G&R est inclus dans un solveur épais tridimensionnel. Dans le nouveau code, l’adaptation est déclenchée à partir de la suppression de la masse, mais dans ce cas, les simulations sont effectuées sur des formes cylindriques, toriques et spécifiques au patient. De ces simulations, on déduit la nécessité d’un pré-étirement non uniforme dans les géométries non cylindriques. Troisièmement, le solveur tridimensionnel comprend une routine pour l’analyse de la propagation d’une dissection aortique dans le cadre de G&R.The aortic wall as other biological systems have adaptations with the purpose to maintain its stability. This stability depends on the cell assessing the state of the aorta. Eventually a disturbed activity of the cells may lead to maladaptations and progression of diseases. Thus, the objective of this thesis is to implement a mechanical approach for the adaptations in a numerical solver and apply it to patient-specific aortas. First, the mechanical approach is included in a two-dimensional axisymmetric shell method. Where the growth and remodeling (G&R) of the tissue is triggered from mass removal or load changes. The load changes are produced by the placement of a stent into the artery and produce further remodeling in it. Second, the G&R model is included within a three-dimensional thick solver. In the new code the adaptation is triggered from mass removal, but in this case the simulations are performed on cylinder, torus and patient-specific shapes. From those simulation is deduced the need for non-uniform prestretch in non-cylindrical geometries. Third, in the three-dimensional solver is included a routine for the analysis of aortic dissection propagation under G&R

Modélisation éléments finis et prédiction personnalisées de la progression de l’anévrisme aortique

The aortic wall as other biological systems have adaptations with the purpose to maintain its stability. This stability depends on the cell assessing the state of the aorta. Eventually a disturbed activity of the cells may lead to maladaptations and progression of diseases. Thus, the objective of this thesis is to implement a mechanical approach for the adaptations in a numerical solver and apply it to patient-specific aortas. First, the mechanical approach is included in a two-dimensional axisymmetric shell method. Where the growth and remodeling (G&R) of the tissue is triggered from mass removal or load changes. The load changes are produced by the placement of a stent into the artery and produce further remodeling in it. Second, the G&R model is included within a three-dimensional thick solver. In the new code the adaptation is triggered from mass removal, but in this case the simulations are performed on cylinder, torus and patient-specific shapes. From those simulation is deduced the need for non-uniform prestretch in non-cylindrical geometries. Third, in the three-dimensional solver is included a routine for the analysis of aortic dissection propagation under G&R.La paroi aortique, comme d’autres systèmes biologiques, présente des adaptations visant à maintenir sa stabilité. Cette stabilité dépend de l’évaluation de l’état de l’aorte par la cellule. A terme, une activité perturbée de les cellules peut conduire à des inadaptations et à la progression de maladies. Ainsi, l’objectif de cette thèse est d’implémenter une approche mécanique des adaptations dans un solveur numérique et de l’appliquer à des aortes spécifiques aux patients. Tout d’abord, l’approche mécanique est incluse dans une méthode de coque axisymétrique bidimensionnelle. La croissance et le remodelage (G&R) du tissu sont déclenchés par l’élimination de la masse ou les changements de charge. Les changements de charge sont produits par le placement d’un stent dans l’artère et produisent un remodelage supplémentaire de celle-ci. Deuxièmement, le modèle G&R est inclus dans un solveur épais tridimensionnel. Dans le nouveau code, l’adaptation est déclenchée à partir de la suppression de la masse, mais dans ce cas, les simulations sont effectuées sur des formes cylindriques, toriques et spécifiques au patient. De ces simulations, on déduit la nécessité d’un pré-étirement non uniforme dans les géométries non cylindriques. Troisièmement, le solveur tridimensionnel comprend une routine pour l’analyse de la propagation d’une dissection aortique dans le cadre de G&R

A new finite‐element shell model for arterial growth and remodeling after stent implantation

International audienceThe goal of this paper is to study computationally how blood vessels adapt when they are exposed to a mechanobiological insult, namely a sudden change of their biomechanical conditions such as proteolytic injuries or implantation. Adaptation occurs through growth and remodeling (G&R), consisting in mass production or removal of structural proteins, such as collagen, until restoring the initial homeostatic biomechanical conditions. In some circumstances, the initial conditions can never be recovered and arteries evolve towards unstable pathological conditions, such as aneurysms, which are responsible for significant morbidity and mortality. Therefore, computational predictions of G&R under different circumstances can be helpful in understanding fundamentally how arterial pathologies progress. For that we have developed a low-cost open-source finite-element 2D axisymmetric shell model (FEM) of the arterial wall. The constitutive equations for static equilibrium used to model the stress-strain behavior and the G&R response are expressed within the homogenized constrained mixture theory. The originality is to integrate the layer-specific behavior of both arterial layers (media and adventitia) into the model. Considering different mechanobiological insults, our results show that the resulting arterial dilatation is strongly correlated with the media thickness. The adaptation to stent implantation is particularly interesting. For large stent over-sizing ratios, the artery cannot recover from the mechanobiological insult and dilates forever, whereas dilatation stabilizes after a transient period for more moderate oversizing ratios. We also show that stent implantation induces a different response in an aneurysm or in a healthy artery, the latter yielding more unstable G&R. Finally, our G&R model can efficiently predict, with very low computational cost, fundamental aspects of arterial adaptation induced by clinical procedures

Patient-specific Finite Element Modeling of Aneurysmal dilatation after chronic type B aortic dissection

International audienceProgressive aneurysmal dilatation is a well-recognized complication in patients with chronic type B aortic dissection (cTBAD), which may lead to a delayed rupture and create a life-threatening condition. However, our understanding of such aortic expansion in cTBAD remains weak. In the present paper, we propose to use numerical simulations to study the role of growth and remodeling (G&R) in aneurysmal dilatation after cTBAD. We set up a 3D finite-element model of G&R for aortic dissection within an open-source code. Constitutive equations, momentum balance equations, and equations related to the mechanobiology of the artery were formulated based on the homogenized constrained mixture theory. The model was first applied to idealized aortic geometries with cylindrical and toric shapes to demonstrate its feasibility and efficiency. The model was then applied to a patient-specific aortic segment to show its potential in more relevant and complex patient-specific clinical applications. It was found that the G&R tends to naturally trigger the aneurysmal dilatation after dissection, in order to restore its tensional equilibrium. Our results indicated that the value of the gain parameter, related to collagen G&R, plays an important role in the stability of aortic expansion after cTBAD. A small gain parameter will induce an excessive aneurysmal degeneration whilst a large gain parameter helps to recover a stabilized state of the artery after dissection. Finally, it was found that other mechanobiology-related parameters, such as the circumferential length of the dissection, as well as the pressure in the false lumen, may also be determinant for the stability of aneurysmal dilatation after cTBAD. Both a wide tear and an elevated false lumen pressure favor an unstable development of aortic expansion after cTBAD. As future work, the present model will be validated through predictions of aneurysmal dilatation in patient-specific clinical cases, in comparison with datasets followed over a significant period of time